Automatic Packet Reporting System (APRS) is an amateur radio based system for real time tactical digital communications of information of immediate value in the local area. In addition, all such data is ingested into the APRS Internet system (APRS-IS) and distributed globally for instant access.

In addition to messages, alerts, announcements and bulletins, the most visible aspect of APRS is its map display. Anyone may place any object or information on their map and it is distributed to all maps of all users in the local RF network or monitoring the area via the internet. Any station, radio or object that has an attached GPS is automatically tracked. Other prominent map features are weather stations, alerts and objects and other map related amateur radio volunteer activities including Search and Rescue and signal direction finding.

APRS was developed since the late 80's by Bob Bruninga, callsign WB4APR, currently a senior research engineer at the United States Naval Academy. The acronym "APRS" was derived from his callsign.  In the 1990's as GPS excitement dominated many new applications, the "P" was often referred to as "Position" instead of the original "Packet". But this so skewed the public perception of APRS as only a GPS and Weather Position tracking system, that recently, the emphasis has returned to the broader "Packet" applications.  (Definition is from Wikipedia.org.  

Click HERE for full description, more links and a lot more information.)

Some additional links  to get you started:

The  place to start learning about APRS is HERE.

A great PowerPoint presentation explaining APRS is HERE. 

A good APRS Tracking site on the  Internet is HERE.  (Fill in the boxes with the  call sign you wish to find and change the time period to see past tracks.)

Byonics.Com   One equipment source..

Information on deviation.